Re: FR + Poison Reverse...
Hey Group, I recently pondered something while at work and nobody could give me a strait answer. Actually I feel a little embarrassed asking this due to me being an NP and I feel like I should know this. Guess this type of thing doesn't come up much around me. Question is: Can poison reverse (PR) be used with OSPF? Not as such, but there is a mechanism that can be used with a similar effect. When a router desires to withdraw an LSA it originated, it sets the age field to a maximum and refloods it. I know split horizon is used with it but I just cant see why/how poison reverse would. My definition of PR is that it sets the link to the max hop count and deems it unreachable, hence the term poison. I can't see how this would work with OSPF because it doesn't use a hop count. Maybe I'm confused about PR. Does it set the link to the highest metric, and not hop count? Maybe I'm just used to hearing about PR in discussions of RIP that I'm assuming it set the hop count to the highest and has nothing to do with metric. Any clarity would help, thanks all... ...sorry for the rambling... _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: FR + Poison Reverse...
But I thought that I'd seen or heard of using split horizon with OSPF...maybe I'm thinking of Frame Relay...long day I guess... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA - Original Message - From: "Brian" [EMAIL PROTECTED] To: "Z" [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Sent: Tuesday, February 27, 2001 1:16 AM Subject: Re: FR + Poison Reverse... split horizon and PR are both associated with distance vector protocols, OSPF is a link state protocol. Brian On Mon, 26 Feb 2001, Z wrote: Hey Group, I recently pondered something while at work and nobody could give me a strait answer. Actually I feel a little embarrassed asking this due to me being an NP and I feel like I should know this. Guess this type of thing doesn't come up much around me. Question is: Can poison reverse (PR) be used with OSPF? I know split horizon is used with it but I just cant see why/how poison reverse would. My definition of PR is that it sets the link to the max hop count and deems it unreachable, hence the term poison. I can't see how this would work with OSPF because it doesn't use a hop count. Maybe I'm confused about PR. Does it set the link to the highest metric, and not hop count? Maybe I'm just used to hearing about PR in discussions of RIP that I'm assuming it set the hop count to the highest and has nothing to do with metric. Any clarity would help, thanks all... ...sorry for the rambling... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] --- I'm buying / selling used CISCO gear!! email me for a quote Brian Feeny,CCDP,CCNP+VAS Scarlett Parria [EMAIL PROTECTED] [EMAIL PROTECTED] 318-222-2638 x 109318-222-2638 x 101 Netjam, LLC http://www.netjam.net 1401 Oden St. Suite 18 Shreveport, LA 71104 Fax 318-221-6612 _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: FR + Poison Reverse...
OSPF does use Split horizon. I don't think OSPF uses PR because PR sets the route to infinity and I'm not sure what an ' infinity' cost would mean in OSPF. -Original Message- From: Z [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 27, 2001 12:47 AM To: Brian; [EMAIL PROTECTED] Subject: Re: FR + Poison Reverse... But I thought that I'd seen or heard of using split horizon with OSPF...maybe I'm thinking of Frame Relay...long day I guess... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA - Original Message - From: "Brian" [EMAIL PROTECTED] To: "Z" [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Sent: Tuesday, February 27, 2001 1:16 AM Subject: Re: FR + Poison Reverse... split horizon and PR are both associated with distance vector protocols, OSPF is a link state protocol. Brian On Mon, 26 Feb 2001, Z wrote: Hey Group, I recently pondered something while at work and nobody could give me a strait answer. Actually I feel a little embarrassed asking this due to me being an NP and I feel like I should know this. Guess this type of thing doesn't come up much around me. Question is: Can poison reverse (PR) be used with OSPF? I know split horizon is used with it but I just cant see why/how poison reverse would. My definition of PR is that it sets the link to the max hop count and deems it unreachable, hence the term poison. I can't see how this would work with OSPF because it doesn't use a hop count. Maybe I'm confused about PR. Does it set the link to the highest metric, and not hop count? Maybe I'm just used to hearing about PR in discussions of RIP that I'm assuming it set the hop count to the highest and has nothing to do with metric. Any clarity would help, thanks all... ...sorry for the rambling... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] --- I'm buying / selling used CISCO gear!! email me for a quote Brian Feeny,CCDP,CCNP+VAS Scarlett Parria [EMAIL PROTECTED] [EMAIL PROTECTED] 318-222-2638 x 109318-222-2638 x 101 Netjam, LLC http://www.netjam.net 1401 Oden St. Suite 18 Shreveport, LA 71104 Fax 318-221-6612 _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: FR + Poison Reverse...
OSPF uses split-horizon? I don't think this is correct. I hate to be quick to reply but there is no reason for OSPF to use split horizon. OSPF sends updates to all adjacent neighbors, and in NBMA , and Broadcast network types the DR forwards the updates on. Split-horizon is used to keep routing loops from happening for a Distance-vector protocol not a link state protocol. There is an issue split-horizon presents when using a distance-vector protocol in a frame-relay hub spoke topology. As you can imagine (if you know how a distance-vector protocol diseminates updates) the hub will need to send the update back out of the interface it received it on so that the other spokes will receive the update. This will not be allowed to happen if split-horizon is enabled. The spokes will not be an issue. By default: Physical interface frame-relay setup split-horizon will be disabled. Subinterface frame-relay setup split-horizon will be enabled. Also keep in mind you cannot disable split-horizon on IPX RIP. Summation when you have a hub spoke frame-relay topology and you are using a distance-vector routing protocol you need to disable split-horizon on the hub. (which does leave you secceptible to the issue split-horizon was designed to fix) Or use a link-state protocol. Or you could setup point-to-point subinterfaces on the hub for each spoke. Brian From: "Maness, Drew" [EMAIL PROTECTED] Reply-To: "Maness, Drew" [EMAIL PROTECTED] To: "'Z'" [EMAIL PROTECTED], Brian [EMAIL PROTECTED], [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Date: Tue, 27 Feb 2001 08:36:57 -0800 OSPF does use Split horizon. I don't think OSPF uses PR because PR sets the route to infinity and I'm not sure what an ' infinity' cost would mean in OSPF. -Original Message- From: Z [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 27, 2001 12:47 AM To: Brian; [EMAIL PROTECTED] Subject: Re: FR + Poison Reverse... But I thought that I'd seen or heard of using split horizon with OSPF...maybe I'm thinking of Frame Relay...long day I guess... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA - Original Message - From: "Brian" [EMAIL PROTECTED] To: "Z" [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Sent: Tuesday, February 27, 2001 1:16 AM Subject: Re: FR + Poison Reverse... split horizon and PR are both associated with distance vector protocols, OSPF is a link state protocol. Brian On Mon, 26 Feb 2001, Z wrote: Hey Group, I recently pondered something while at work and nobody could give me a strait answer. Actually I feel a little embarrassed asking this due to me being an NP and I feel like I should know this. Guess this type of thing doesn't come up much around me. Question is: Can poison reverse (PR) be used with OSPF? I know split horizon is used with it but I just cant see why/how poison reverse would. My definition of PR is that it sets the link to the max hop count and deems it unreachable, hence the term poison. I can't see how this would work with OSPF because it doesn't use a hop count. Maybe I'm confused about PR. Does it set the link to the highest metric, and not hop count? Maybe I'm just used to hearing about PR in discussions of RIP that I'm assuming it set the hop count to the highest and has nothing to do with metric. Any clarity would help, thanks all... ...sorry for the rambling... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] --- I'm buying / selling used CISCO gear!! email me for a quote Brian Feeny,CCDP,CCNP+VAS Scarlett Parria [EMAIL PROTECTED] [EMAIL PROTECTED] 318-222-2638 x 109318-222-2638 x 101 Netjam, LLC http://www.netjam.net 1401 Oden St. Suite 18 Shreveport, LA 71104 Fax 318-221-6612 _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ Get your FREE download of MSN Explorer at http://explorer.msn.com _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: FR + Poison Reverse...
After verifying my thoughts with referrences I am now confident OSPF does not in any way use split-horizon. I was a little leary to be absolutely sure right away, since there are often nuances that exist. Split-horizon, Poison Reverse, and Hold down timers were created to overcome loops caused by the way Distance-vector protocols converge. Brian From: "Maness, Drew" [EMAIL PROTECTED] Reply-To: "Maness, Drew" [EMAIL PROTECTED] To: "'Z'" [EMAIL PROTECTED], Brian [EMAIL PROTECTED], [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Date: Tue, 27 Feb 2001 08:36:57 -0800 OSPF does use Split horizon. I don't think OSPF uses PR because PR sets the route to infinity and I'm not sure what an ' infinity' cost would mean in OSPF. -Original Message- From: Z [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 27, 2001 12:47 AM To: Brian; [EMAIL PROTECTED] Subject: Re: FR + Poison Reverse... But I thought that I'd seen or heard of using split horizon with OSPF...maybe I'm thinking of Frame Relay...long day I guess... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA - Original Message - From: "Brian" [EMAIL PROTECTED] To: "Z" [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Sent: Tuesday, February 27, 2001 1:16 AM Subject: Re: FR + Poison Reverse... split horizon and PR are both associated with distance vector protocols, OSPF is a link state protocol. Brian On Mon, 26 Feb 2001, Z wrote: Hey Group, I recently pondered something while at work and nobody could give me a strait answer. Actually I feel a little embarrassed asking this due to me being an NP and I feel like I should know this. Guess this type of thing doesn't come up much around me. Question is: Can poison reverse (PR) be used with OSPF? I know split horizon is used with it but I just cant see why/how poison reverse would. My definition of PR is that it sets the link to the max hop count and deems it unreachable, hence the term poison. I can't see how this would work with OSPF because it doesn't use a hop count. Maybe I'm confused about PR. Does it set the link to the highest metric, and not hop count? Maybe I'm just used to hearing about PR in discussions of RIP that I'm assuming it set the hop count to the highest and has nothing to do with metric. Any clarity would help, thanks all... ...sorry for the rambling... This has been an Eyez Only streaming e-mail broadcast...We are watching. NetEyez ~ CCNP, CCDA _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] --- I'm buying / selling used CISCO gear!! email me for a quote Brian Feeny,CCDP,CCNP+VAS Scarlett Parria [EMAIL PROTECTED] [EMAIL PROTECTED] 318-222-2638 x 109318-222-2638 x 101 Netjam, LLC http://www.netjam.net 1401 Oden St. Suite 18 Shreveport, LA 71104 Fax 318-221-6612 _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ Get your FREE download of MSN Explorer at http://explorer.msn.com _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: FR + Poison Reverse...
If you don't want to read all of my quotes, look for the portion contained therein enclosed by double asterisks (**). From Cisco's site: "Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks, such as Frame Relay and SMDS, situations can arise for which this behavior is less than ideal. For these situations, you might want to disable split horizon. **This applies to IGRP and RIP.**" This excerpt can be found at: http://www.cisco.com/univercd/cc/td/doc/product/software/ios11/cbook/ciproute.htm#xtocid16743169 A few weeks ago, this same discussion was brought up. Someone posted the following link outlining "Hub And Spoke Frame Relay Sample Configuration - Dynamic (OSPF) IP Routing". Here's an excerpt from the link: "In general, it is good practice to use subinterfaces for partially-meshed frame relay networks. A frame relay network designed with subinterfaces scales much easier to future expansion. Referring to the example, subinterfaces allow routing updates to exchange between Boston and Chicago through Atlanta. **Without subinterfaces, Boston is unable to receive routing updates from Chicago and vice versa creating a condition known as split-horizon.**" This excerpt can be found at: http://www.cisco.com/warp/public/779/smbiz/service/configs/framerelay/fr_ip_ospf.htm Now this is in contrast to what I know of OSPF and in contrast to the first quote I've included. Quite frankly, I would think that the problems encountered in the Frame Relay example have more to do with the non-broadcast nature of Frame Relay than split-horizon so I think whomever wrote this may have misunderstood why subinterfaces enable routing updates--and I think perhaps this is why there's so much confusion over whether or not split-horizon plays any role in OSPF. Brian L: I think you've got it backwards as to when split-horizon operates on an interface. By default, all Cisco serial interfaces are multipoint unless specifically configured to be point-to-point. With multipoint, you'd want split-horizon enabled if you were using a broadcasting routing protocol. Here's two excerpts from Cisco's site: --Excerpt 1-- "Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame relay interfaces and multipoint subinterfaces and do this by default. However, split-horizon cannot be disabled for other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired." --Excerpt 2-- "Cisco serial interfaces are multipoint interfaces by default unless specified as a point-to-point subinterface. Though less common than point-to-point subinterfaces, it is possible to divide the interface into separate virtual multipoint subinterfaces." "Multipoint interfaces/subinterfaces are still subject to the split-horizon limitations as discussed above. All nodes attached to a multipoint subinterface belong to the same network number. Typically, multipoint subinterfaces are used in conjunction with point-to-point interfaces in cases where an existing multipoint frame relay cloud is migrating to a subinterfaced point-to-point network design. A multipoint subinterface is used to keep remote sites on a single network number while slowly migrating remote sites to their own point-to-point subinterface network." "Figure 4 shows serial 0.1 as a multipoint subinterface connecting to three different locations. All devices on the multipoint subinterface belong to the same network number (100). Site E has migrated off of the multipoint network to its own point-to-point subinterface network (200). Eventually, all remote sites can be moved to their own point-to-point subinterface networks and the multipoint subinterface will not be necessary." Both of these excerpts can be found at: http://www.cisco.com/warp/public/779/smbiz/service/knowledge/wan/subifs.htm -- Leigh Anne -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Brian Lodwick Sent: February 27, 2001 10:21 AM To: [EMAIL PROTECTED]; [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... OSPF uses split-horizon? I don't think this is correct. I hate to be quick to reply but there is no reason for OSPF to use split horizon. OSPF sends updates to all adjacent neighbors, and in NBMA , and Broadcast network types the DR forwards the updates on. Split-horizon is used to keep routing loops from happening for a Distance-vector protocol not a link state protocol. There is
RE: FR + Poison Reverse...
Leigh Anne Chisholm writes: Brian L: I think you've got it backwards as to when split-horizon operates on an interface. By default, all Cisco serial interfaces are multipoint unless specifically configured to be point-to-point. With multipoint, you'd want split-horizon enabled if you were using a broadcasting routing protocol. --Excerpt 1-- "Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame relay interfaces and multipoint subinterfaces and do this by default. However, split-horizon cannot be disabled for other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired." Brian's reply: You have just cut from a cisco document that proves I am accurate. This says all interfaces are by default multipoint unless specifically configured point-to-point. This is talking about physical interface configuration not multipoint subinterface configuration, and you have just proved I am correct that Physical interfaces configured for frame-relay by default disable split-horizon. Also if you want a distance-vector protocol to work correctly on a Frame-relay hub spoke model split-horizon must be disabled on the hub(unless the hub is configured for point-to-point to each spoke) Brian From: "Leigh Anne Chisholm" [EMAIL PROTECTED] Reply-To: [EMAIL PROTECTED] To: "Brian Lodwick" [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Date: Tue, 27 Feb 2001 11:03:21 -0700 If you don't want to read all of my quotes, look for the portion contained therein enclosed by double asterisks (**). From Cisco's site: "Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks, such as Frame Relay and SMDS, situations can arise for which this behavior is less than ideal. For these situations, you might want to disable split horizon. **This applies to IGRP and RIP.**" This excerpt can be found at: http://www.cisco.com/univercd/cc/td/doc/product/software/ios11/cbook/ciproute.htm#xtocid16743169 A few weeks ago, this same discussion was brought up. Someone posted the following link outlining "Hub And Spoke Frame Relay Sample Configuration - Dynamic (OSPF) IP Routing". Here's an excerpt from the link: "In general, it is good practice to use subinterfaces for partially-meshed frame relay networks. A frame relay network designed with subinterfaces scales much easier to future expansion. Referring to the example, subinterfaces allow routing updates to exchange between Boston and Chicago through Atlanta. **Without subinterfaces, Boston is unable to receive routing updates from Chicago and vice versa creating a condition known as split-horizon.**" This excerpt can be found at: http://www.cisco.com/warp/public/779/smbiz/service/configs/framerelay/fr_ip_ospf.htm Now this is in contrast to what I know of OSPF and in contrast to the first quote I've included. Quite frankly, I would think that the problems encountered in the Frame Relay example have more to do with the non-broadcast nature of Frame Relay than split-horizon so I think whomever wrote this may have misunderstood why subinterfaces enable routing updates--and I think perhaps this is why there's so much confusion over whether or not split-horizon plays any role in OSPF. Brian L: I think you've got it backwards as to when split-horizon operates on an interface. By default, all Cisco serial interfaces are multipoint unless specifically configured to be point-to-point. With multipoint, you'd want split-horizon enabled if you were using a broadcasting routing protocol. Here's two excerpts from Cisco's site: --Excerpt 1-- "Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame relay interfaces and multipoint subinterfaces and do this by default. However, split-horizon cannot be disabled for other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired." --Excerpt 2-- "Cisco serial interfaces are multipoint interfaces by default unless specified as a point-to-point subinterface. Though less common than point-to-point subinterfaces, it is possible to divide the interface into separate virtual multipoint subinterfaces." "Multipoint interfaces/subinterfaces are still subject to the split-horizon limitations as discussed above. All nodes attached to a multipoint subinterface belong to the same network number. Typically, mul
RE: FR + Poison Reverse...
Whoops! When I posted Excerpt 1, I missed the part in the first sentence that seems to imply that frame-relay interfaces and multipoint interfaces disable split-horizon BY DEFAULT (when using TCP/IP). When I took ICRC so many years ago, I remember discussing split-horizon but don't recall any explicit mention that split-horizon is disabled by default for the IP routing protocols (RIP and IGRP). I remember discussing this issue as well in CIT but I don't think the exception to the rule was noted either--but because I wasn't aware of it, it may have been something that went in one ear and out the other. It's interesting to note that in the current ICND curriculum, I know the section that talks about configuring subinterfaces indicates that "multipoint subinterfaces act as NBMA network so they do not resolve the split horizon issue". I don't have the entire official ICND curriculum available, but I don't notice anything specifically mentioning the exception. So, in summary - we're both right with respect to multipoint interfaces: With multipoint (or physical) interfaces: split horizon is ENABLED by default unless you are routing IP across the link. Then it is DISABLED. If you're routing using RIP, IGRP, or EIGRP to route IP, it's DISABLED. If you're routing IPX using RIP, EIGRP, NLSP, it's ENABLED. It's also enabled if routing AppleTalk. With point-to-point subinterfaces, split horizon is ENABLED but isn't much of an issue since it's a point-to-point link. -- Leigh Anne -Original Message- From: Brian Lodwick [mailto:[EMAIL PROTECTED]] Sent: February 27, 2001 11:26 AM To: [EMAIL PROTECTED]; [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Leigh Anne Chisholm writes: Brian L: I think you've got it backwards as to when split-horizon operates on an interface. By default, all Cisco serial interfaces are multipoint unless specifically configured to be point-to-point. With multipoint, you'd want split-horizon enabled if you were using a broadcasting routing protocol. --Excerpt 1-- "Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame relay interfaces and multipoint subinterfaces and do this by default. However, split-horizon cannot be disabled for other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired." Brian's reply: You have just cut from a cisco document that proves I am accurate. This says all interfaces are by default multipoint unless specifically configured point-to-point. This is talking about physical interface configuration not multipoint subinterface configuration, and you have just proved I am correct that Physical interfaces configured for frame-relay by default disable split-horizon. Also if you want a distance-vector protocol to work correctly on a Frame-relay hub spoke model split-horizon must be disabled on the hub(unless the hub is configured for point-to-point to each spoke) Brian From: "Leigh Anne Chisholm" [EMAIL PROTECTED] Reply-To: [EMAIL PROTECTED] To: "Brian Lodwick" [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Date: Tue, 27 Feb 2001 11:03:21 -0700 If you don't want to read all of my quotes, look for the portion contained therein enclosed by double asterisks (**). From Cisco's site: "Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks, such as Frame Relay and SMDS, situations can arise for which this behavior is less than ideal. For these situations, you might want to disable split horizon. **This applies to IGRP and RIP.**" This excerpt can be found at: http://www.cisco.com/univercd/cc/td/doc/product/software/ios11/cbook/ciproute .htm#xtocid16743169 A few weeks ago, this same discussion was brought up. Someone posted the following link outlining "Hub And Spoke Frame Relay Sample Configuration - Dynamic (OSPF) IP Routing". Here's an excerpt from the link: "In general, it is good practice to use subinterfaces for partially-meshed frame relay networks. A frame relay network designed with subinterfaces scales much easier to future expansion. Referring to the example, subinterfaces allow routing updates to exchange between Boston and Chicago through Atlanta. **Without subinterfaces, Boston is unable to receive routing updates from Chicago and vice versa creating a condition known as split-horizon.**" This excerpt can be found at: http://
RE: FR + Poison Reverse...
Judging by the web page on Cisco's site (http://www.cisco.com/warp/public/779/smbiz/service/knowledge/wan/subifs.htm), it appears as if split-horizon is disabled for routing IP on physical (multipoint) interfaces as well as multipoint subinterfaces. Here's the part I'm referring to: "Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame relay interfaces and multipoint subinterfaces and do this by default. However, split-horizon cannot be disabled for other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired." Out of curiosity, what is it that leads you to believe that split-horizon is enabled on frame relay subinterfaces? -- Leigh Anne -Original Message- From: Brian Lodwick [mailto:[EMAIL PROTECTED]] Sent: February 27, 2001 1:45 PM To: [EMAIL PROTECTED]; [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Leigh Anne Chisholm, Actually by default on IP interfaces: Split-horizon is disabled on Frame-relay physical interfaces Split-horizon is enabled on Frame-relay point-to-point and multipoint subinterfaces (but who cares it's configurable) There is a difference between a multipoint physical interface and a multipoint subinterface. Althought there aren't too many differences (static mapping and dynamic mapping allowed on both) there are subtle differences. My brother and I went over this yesterday and wondered what the differences are. What we came down to was that the big difference is simple -subinterfaces function is to provide a means to logically separate a physical interface. The subtle differences like the default split-horizon setting and default OSPF network type aren't the big differences. You can build a fully scaleable hub spoke frame model using a physical multipoint interface on the hub and point-to-point subinterfaces on the spokes. The advantage of using the Multipoint subinterface on the hub? You would only need 1 physical interface for 2 hub spoke frame models. I am interested in learning more about other things you can do with the hub as a multipoint subinterface to control traffic, such as for filtering or traffic shaping. Brian From: "Leigh Anne Chisholm" [EMAIL PROTECTED] Reply-To: [EMAIL PROTECTED] To: "Brian Lodwick" [EMAIL PROTECTED], "Cisco@Groupstudy. Com" [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Date: Tue, 27 Feb 2001 12:41:07 -0700 Whoops! When I posted Excerpt 1, I missed the part in the first sentence that seems to imply that frame-relay interfaces and multipoint interfaces disable split-horizon BY DEFAULT (when using TCP/IP). When I took ICRC so many years ago, I remember discussing split-horizon but don't recall any explicit mention that split-horizon is disabled by default for the IP routing protocols (RIP and IGRP). I remember discussing this issue as well in CIT but I don't think the exception to the rule was noted either--but because I wasn't aware of it, it may have been something that went in one ear and out the other. It's interesting to note that in the current ICND curriculum, I know the section that talks about configuring subinterfaces indicates that "multipoint subinterfaces act as NBMA network so they do not resolve the split horizon issue". I don't have the entire official ICND curriculum available, but I don't notice anything specifically mentioning the exception. So, in summary - we're both right with respect to multipoint interfaces: With multipoint (or physical) interfaces: split horizon is ENABLED by default unless you are routing IP across the link. Then it is DISABLED. If you're routing using RIP, IGRP, or EIGRP to route IP, it's DISABLED. If you're routing IPX using RIP, EIGRP, NLSP, it's ENABLED. It's also enabled if routing AppleTalk. With point-to-point subinterfaces, split horizon is ENABLED but isn't much of an issue since it's a point-to-point link. -- Leigh Anne _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: FR + Poison Reverse...
OK, looks like you are right according to that. I was reading from Bruce Caslow's book. I'll go with Cisco. Thanks for the correspondance Leigh Anne! Brian From: "Leigh Anne Chisholm" [EMAIL PROTECTED] Reply-To: [EMAIL PROTECTED] To: "Brian Lodwick" [EMAIL PROTECTED], [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Date: Tue, 27 Feb 2001 14:44:28 -0700 Judging by the web page on Cisco's site (http://www.cisco.com/warp/public/779/smbiz/service/knowledge/wan/subifs.htm), it appears as if split-horizon is disabled for routing IP on physical (multipoint) interfaces as well as multipoint subinterfaces. Here's the part I'm referring to: "Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame relay interfaces and multipoint subinterfaces and do this by default. However, split-horizon cannot be disabled for other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired." Out of curiosity, what is it that leads you to believe that split-horizon is enabled on frame relay subinterfaces? -- Leigh Anne -Original Message- From: Brian Lodwick [mailto:[EMAIL PROTECTED]] Sent: February 27, 2001 1:45 PM To: [EMAIL PROTECTED]; [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Leigh Anne Chisholm, Actually by default on IP interfaces: Split-horizon is disabled on Frame-relay physical interfaces Split-horizon is enabled on Frame-relay point-to-point and multipoint subinterfaces (but who cares it's configurable) There is a difference between a multipoint physical interface and a multipoint subinterface. Althought there aren't too many differences (static mapping and dynamic mapping allowed on both) there are subtle differences. My brother and I went over this yesterday and wondered what the differences are. What we came down to was that the big difference is simple -subinterfaces function is to provide a means to logically separate a physical interface. The subtle differences like the default split-horizon setting and default OSPF network type aren't the big differences. You can build a fully scaleable hub spoke frame model using a physical multipoint interface on the hub and point-to-point subinterfaces on the spokes. The advantage of using the Multipoint subinterface on the hub? You would only need 1 physical interface for 2 hub spoke frame models. I am interested in learning more about other things you can do with the hub as a multipoint subinterface to control traffic, such as for filtering or traffic shaping. Brian From: "Leigh Anne Chisholm" [EMAIL PROTECTED] Reply-To: [EMAIL PROTECTED] To: "Brian Lodwick" [EMAIL PROTECTED], "Cisco@Groupstudy. Com" [EMAIL PROTECTED] Subject: RE: FR + Poison Reverse... Date: Tue, 27 Feb 2001 12:41:07 -0700 Whoops! When I posted Excerpt 1, I missed the part in the first sentence that seems to imply that frame-relay interfaces and multipoint interfaces disable split-horizon BY DEFAULT (when using TCP/IP). When I took ICRC so many years ago, I remember discussing split-horizon but don't recall any explicit mention that split-horizon is disabled by default for the IP routing protocols (RIP and IGRP). I remember discussing this issue as well in CIT but I don't think the exception to the rule was noted either--but because I wasn't aware of it, it may have been something that went in one ear and out the other. It's interesting to note that in the current ICND curriculum, I know the section that talks about configuring subinterfaces indicates that "multipoint subinterfaces act as NBMA network so they do not resolve the split horizon issue". I don't have the entire official ICND curriculum available, but I don't notice anything specifically mentioning the exception. So, in summary - we're both right with respect to multipoint interfaces: With multipoint (or physical) interfaces: split horizon is ENABLED by default unless you are routing IP across the link. Then it is DISABLED. If you're routing using RIP, IGRP, or EIGRP to route IP, it's DISABLED. If you're routing IPX using RIP, EIGRP, NLSP, it's ENABLED. It's also enabled if routing AppleTalk. With point-to-point subinterfaces, split horizon is ENABLED but isn't much of an issue since it's a point-to-point link. -- Leigh Anne _ Get your FREE download of MSN Explorer at http://explorer.msn.com _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
