To conclude this introduction to BGP let's have a look at how the router displays the BGP status and the BGP table. First off we'll have a look at IPv4. This is on Cisco IOS and the summary displayed on the screen in front of you shows quite a lot of useful information about the BGP neighbor status. First off you have a column listing the IP addresses of the neighbors. It then displays the BGP version number. We use BGP version 4 today as you have already learned. And then lists the autonomous system number, the messages received and sent. So these are the updates that have been sent by the router and received by the router from each individual neighbor. The table version indicates any changes. So it's an incremental counter of the changes in the BGP table. And then you have two columns showing the input queue and output queue; the queues of the updates which are waiting to be processed. These queues are normally 0 but if the router is under load or you're trying to transfer a large BGP table you'll sometimes see values in these queues. The next column shows how long the peering is up or shows how long the peering is down. And the state up or down is shown in the final column. If you see numbers there that shows how many prefixes have been received. If you see status messages like "no connect" and so forth then the peering is considered done. So the screen in front of you shows the IPv4 version of this. For IPv6 the display is actually very similar again we have a list of the neighbors. This time the v6 addresses as well as BGP version number, AS number, and all the different messages, and the number of prefixes received. If you look near the top of the router display it also indicates things like the BGP router identifier which is a unique 32-bit integer identifying the router as well as the AS number and various other details about how much memory is needed, how many network entries there are and so forth. And then looking at the next table on the screen in front of you the "show ip bgp" output displays the BGP table and this is an excerpt from an active router in one of the technical workshops that we offer. In this output you see the list of prefixes that have been learned, the IP addresses of the Next Hop of the routers these will learn from and various other BGP attributes like metric, local preference, weight and path which we will cover going forwards in this series. The screen shows you the IPv4 version and IPv6 version has similar information but of course for IPv6 adjacencies and IPv6 network entries.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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