Blog Posts in December 2007
Sometimes you'll see a weird route status (RIB-failure) in your BGP table, for example:
GW#show ip bgp ¦ include r>
r> 10.2.0.0/16 10.0.1.2 0 0 65001 i
A more thorough investigation of the BGP entry does not give you a lot of additional information:
GW#show ip bgp 10.2.0.0
BGP routing table entry for 10.2.0.0/16, version 7
Paths: (1 available, best #1, table Default-IP-Routing-Table, RIB-failure(17))
Advertised to update-groups:
10.0.1.2 from 10.0.1.2 (10.0.1.2)
Origin IGP, metric 0, localpref 100, valid, external, best
The “mistery” is solved when you inspect the entry in the IP routing table:
GW#show ip route 10.2.0.0
Routing entry for 10.2.0.0/16
Known via "static", distance 1, metric 0 (connected)
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 0, traffic share count is 1
The GW router has a static route that collides with the EBGP route and thus the BGP route cannot be inserted in the IP routing table (as the static route has administrative distance 1).
Let's conclude with a few interesting facts about the RIB failures:
- The RIB failure feature was introduced in IOS release 12.2T; prior to that, the BGP routes with higher administrative distance than other route sources were silently ignored (similar to all other routing protocols).
- You can display BGP routes that are not inserted in the IP routing table with the show ip bgp rib-failure command, which also explains why the BGP route was not inserted in the IP routing table.
- The BGP routes that are not used due to higher administrative distance are still advertised to all BGP peers (contrary to what most other distance-vector routing protocols do), unless you configure bgp suppress-inactive (introducted in 12.2T and 12.0(26)S).
event manager applet NoReload… you cannot enter the action x.y reload configuration command any more (or any other command that includes the string reload).
event cli pattern "reload" sync no skip yes
To distinguish the reload command from other appearances of the same string, use the ^reload pattern (reload occuring at the beginning of the line).
Trivia: this actually occured to me when I was testing the setup described in the December IP Corner article. Sometimes we have to learn the hard way :)
Occasionally someone would try to persuade me that the layer-2 VPN services are like aspirin (you know, totally harmless plus it could get rid of all your headaches). OK, that might be true if you take the layer-2 VPN offering as a pure transport solution and plug in an extra router (sometimes also called a layer-3 switch by marketing people) between the Service Provider’s Ethernet (or whatever they give you) and your LAN. But there are people who don’t know the details and plug the SP Ethernet straight into their L2 switch … and things might even work for a while … until the whole network collapses.
In my opinion, we need both L2 and L3 VPN services, but it’s important that they are positioned and deployed correctly. You can read more about my views on this topic in the SearchTelecom article Making the case for Layer 2 and Layer 3 VPNs.
I remember being involved in a situation years ago (around the 12.0T release) where someone wanted to use MPLS TE without IS-IS (which was the only supported protocol in those days) and somehow the solution was to set up tunnels using explicit paths, where you have to specify hop-by-hop IP addresses. When you think about it, it makes perfect sense: if you list every IP address in the path, there is no need for constraint-based path calculation (PCALC). However, as it turns out, the later additions to MPLS TE (loose source routing, address exclusion, inter-area MPLS TE, inter-AS MPLS TE) changed the IOS code sufficiently that even the hop-by-hop tunnels cannot be set up without operational OSPF or IS-IS:
- In order to have MPLS TE running on a router, you need an MPLS TE router-id, and you can only specify that in OSPF or IS-IS routing protocol.
- Even though the hop-by-hop explicit path is static, the router wants to run PCALC for every hop in the path. If the next-hop IP address is not in the OSPF topology database, the router will not even try to set up the tunnel.
If you want to run MPLS TE in your network, you thus need to run OSPF or IS-IS, even though you might not want to use them for IP packet forwarding. For example, you could enable one of them only on the links actually used for MPLS TE and set the distance to 255 to prevent their routes from getting into the IP routing table (and I've tested it in the lab before writing this post).
The action cli commands used in EEM applets as well as the cli* Tcl functions used in EEM Tcl policies open a virtual Telnet session to a VTY line to execute the CLI commands. The first command you have to execute in the EEM applet is thus the enable command to ensure the next commands will be executed with privilege level 15.
You don't have to specify the enable password.
Likewise, if you want to configure the router, the next command to execute is the configure terminal command, followed by the configuration commands.
03:28:42: RT: add 10.0.3.3/32 via 10.2.0.2, ospf metric [110/75]
03:28:42: RT: NET-RED 10.0.3.3/32
03:28:44: RT: Try lookup less specific 10.0.3.3/32, default 1
03:28:44: RT: Found subnet on less specific 10.0.3.3/32
03:28:44: %BGP-5-ADJCHANGE: neighbor 10.0.3.3 Up
By default, the EEM action cli command waits until it receives exec-level prompt from the VTY (Router> or Router#), resulting in an endless wait and aborted EEM applet in IOS release 12.4(15)T (earlier releases would hang a VTY line forever) if a CLI command returns an additional prompt. With the pattern option, you can change the expected reply to whatever prompt the CLI command is outputting.
As I know some of you run large networks, could you help me understand what you're using (without giving away too much information, of course):
- Are you running a BGP network without MPLS or are you using BGP on the edges and MPLS transport in the core?
- If you have a large number of BGP routers, do you have a nice hierarchy of BGP route reflectors (or confederations) or ad-hoc implementation where every router has all neighbors as RR-clients?
Full disclosure: I might use the information you give me in an upcoming article.