> Hello Murtaza, > My problem was that the core router was running BGP (simulated ISP > connection) and I also had the 0.0.0.0 0.0.0.0 s0 route and I redistributed > this into eigrp so as to inject a default route of D EX to other routers in > my EIGRP domain. > Question is: Would this default route on my core router be used for all > outing connections to the Internet? >Wouldn't this also override BGP as it > has a AD of 0 compared to that of 20 of BGP? > Please advise
Good question. Following are my two cents in this regard. NO. If you are using BGP and needs to go to network that is known via BGP then for that route there will be an entry in the routing table with the configured next hop, i.e.,IP address of the ISP.Assuming that you have already configured the EBGP on your core. In case the destination network is not known then the gateway of last resort will be used which will forward the packet to s0 interface. In this scenario even if we are using the same serial interface to reach the ISP, the above explanation is for understanding the way the route is selected. In both the cases s0 will be used. (Normally in BGP traffic to 0/0 network is forwarded to null0 to avoid routing loops.) Normally, either you run static route to reach the ISP, or use IGP or use BGP. So, running BGP and static route together is not very common. Since, we are now talking BGP on the core, anytime internal routers are following defaults to reach unknown to the AS, we should be careful not to create routing loops. A default route is injected differently from BGP into IGP, depending on what IGP you are using. BGP learned defaults are injected into EIGRP via redistribution. We can inject only the default routes into IGP using route filtering using a route map. Any other updates should be prevented from being redistributed inside IGP. The administrative distances comes into play when we are trying to select the sources of routing information. If we are learning the same route from two different routing protocols then the one with the lowest administrative distance will win, if it has the longest match. Some more info from cisco'site: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm#xtocid20439 30 Administrative Distance Normally, a route could be learned via more than one protocol. Administrative distance is used to discriminate between routes learned from more than one protocol. The route with the lowest administrative distance is installed in the IP routing table. By default, BGP uses the administrative distances shown in Table 12-2. Table 12-2. BGP Default Distances Distance Default Value Function External 20 Applied to routes learned from EBGP Internal 200 Applied to routes learned from IBGP Local 200 Applied to routes originated by the router Note Distance does not influence the BGP path selection algorithm, but it does influence whether BGP-learned routes are installed in the IP routing table. > Thank you. ----- Original Message ----- From: "Cisco Nuts" To: Sent: Monday, January 14, 2002 11:44 AM Subject: Re: EIGRP 'default network' Rehash [7:31799] > Hello Murtaza, > My problem was that the core router was running BGP (simulated ISP > connection) and I also had the 0.0.0.0 0.0.0.0 s0 route and I redistributed > this into eigrp so as to inject a default route of D EX to other routers in > my EIGRP domain. > Question is: Would this default route on my core router be used for all > outing connections to the Internet? Wouldn't this also override BGP as it > has a AD of 0 compared to that of 20 of BGP? > Please advise. > Thank you. > > > >From: "Murtaza Syed" > >Reply-To: "Murtaza Syed" > >To: [EMAIL PROTECTED] > >Subject: Re: EIGRP 'default network' Rehash [7:31799] > >Date: Mon, 14 Jan 2002 01:37:29 -0500 > > > >Following is some info from Cisco's site: > >http://www.cisco.com/warp/public/103/eigrp8.html > > > >Default Routing > > > >There are two ways to inject a default route into EIGRP: redistribute a > >static route or summarize to 0.0.0.0/0. Use the first method when > >you want to draw all traffic to unknown destinations to a default route at > >the core of the network. This method is effective for advertising > >connections to the Internet. For example: > > > > ip route 0.0.0.0 0.0.0.0 x.x.x.x (next hop to the internet) > > ! > > router eigrp 100 > > redistribute static > > default-metric 10000 1 255 1 1500 > > > >The static route that is redistributed into EIGRP does not have to be to > >network 0.0.0.0. If you use another network, you must use the ip > >default-network command to mark the network as a default network. Please > >refer to Configuring a Gateway of Last Resort for further > >information. > > > >Summarizing to a default route is effective only when you want to provide > >remote sites with a default route. Since summaries are configured > >per interface, you don't need to worry about using distribute-lists or > >other > >mechanisms to prevent the default route from being propagated > >toward the core of your network. Note that a summary to 0.0.0.0/0 overrides > >a default route learned from any other routing protocol. The > >only way to configure a default route on a router using this method is to > >configure a static route to 0.0.0.0/0. (Beginning in Cisco IOS > >Software 12.0(4)T, you can also configure an administrative distance on the > >end of the summary-address command, so the local summary > >doesn't override the 0.0.0.0/0 route). > > > > router eigrp 100 > > network 10.0.0.0 > > ! > > interface serial 0 > > encapsulation frame-relay > > no ip address > > ! > > interface serial 0.1 point-to-point > > ip address 10.1.1.1 > > frame-relay interface-dlci 10 > > ip summary-address eigrp 100 0.0.0.0 0.0.0.0 > > > >Regards, > > > >Murtaza > > > >----- Original Message ----- > >From: "s vermill" > >To: > >Sent: Sunday, January 13, 2002 4:33 PM > >Subject: EIGRP 'default network' Rehash [7:31799] > > > > > > > Sorry to bring this up again but apparently, for at least some of us, it > > > needs to be. I chimed in the other day and offered a way to get EIGRP > >to > > > distribute a default route. That basically amounted to simply defining > >a > > > static to 0.0.0.0 and redistributing static into EIGRP (whoever > >corrected > >me > > > by pointing out that the 'network 0.0.0.0' command isn't necessary, I > >thank > > > you (it was in BSCN)). > > > > > > Wayne jumped in and explained some problems with EIGRP and the 'ip > > > default-network' command. I thought it all sounded quite reasonable. > >So > >I > > > tried this in the lab for several hours today (no "get a life" remarks > > > please). I must be really dense, because nothing I try works quite the > >way > > > (that I thought) it was described. It seems that the only way to > > > sucessfully use the default-network is to configure it on every single > > > router in the AS (I tried this with RIP just as a sanity check and it > >worked > > > just fine). > > > > > > If that is the case, can a good argument be made in favor of this > >approach > > > over redistributing static? Or summarizing to 0.0.0.0 0.0.0.0 on an > > > interface? Both of those, at least, propogate throughout the AS after > > > configuration on just one router. > > > > > > Thanks in advance, > > > > > > Scott > _________________________________________________________________ > Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=32106&t=32106 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
