Per popular request, this post has the purpose to explain the solution of the problem of BGP peering with an an unknown AS. I would like to start by stating the obvious.

This is a solution in a search of a problem. Popular as it may be with CCIE workbook vendors and rumor has it with CCIE exam authors, real life has almost no use of this. When faced with a log message that you will read about in few paragraphs, solution is using the phone and not doing the other stuff I’m about to write. That said, it’s an interesting and fun problem.

Read Full Entry »

driymcg3h2

BGP Outbound Route Filtering

R1 and R2 are EBGP Neighbors. R1 is receiving the following routes from R2:

R1#sho ip bgp | be Net
Network          Next Hop            Metric LocPrf Weight Path
*> 200.0.0.0        192.168.12.2             0             0 200 i
*> 200.0.1.0        192.168.12.2             0             0 200 i
*> 200.0.2.0        192.168.12.2             0             0 200 i
*> 200.0.3.0        192.168.12.2             0             0 200 i Read Full Entry  »

The table-map command is a lesser known and sometimes misunderstood BGP configuration command that can be used to set metrics or route-tags on routes. Here we use it to set a route-tag and verify that the tag stays after redistribution into OSPF.

R2 (AS #2) and R3 (AS #3) are BGP Peers. R3 and R4 are OSPF neighbors. R2 is advertising 192.168.12.0 and R3 is redistributing it into OSPF.

Read Full Entry »

One of my students was talking to me today about BGP and various methods with which we can allow connectivity from within an IBGP cloud to external networks.

Generally speaking, for EBGP peering using the BGP standards, we peer based on the IP address of the BGP neighbors’ local interfaces. However, that brings up some challenging permutations. When peering is done via the external network of a BGP edge router within an AS, a BGP edge router will not change the next hop of for the route from the advertising AS when it receives it but rather will pass it on unchanged to its IBGP neighbors.

If the external network connecting the two AS’s is not advertised into the IGP of the receiving AS, the next hop of the route will presumably be unreachable.

Read Full Entry »

The Service Provider CCIE track offers a number of enticements to CCIE candidates. However, it is one of the most overlooked tracks, comprising only about 6% of CCIE numbers (about half as many as Security) and only 2% of R&S CCIEs undertake it successfully.

Looking at these numbers, I find that quite an oddity. Obviously, R&S is by far the most popular track and, like most people, it was the first that I undertook. By far the greatest intellectual achievement of my lifetime was obtaining my first CCIE. However, having done so like many people with the study bug, it is worth considering what to study next.

Read Full Entry »