Things to know… (written)

Braindump for R&S written [2009-01-07: has been taken down, link was]

Erlang B

Erlang B ia an algorithm used to handle trunk allocation.
In the circuit switch environment, many of the larger switches were blocking other switches. That is, not every input had a path to every output. Complex grading structures were created to help determine the pathways a circuit might take through the switch, and the impact on the GoS.

It should be recognized that traffic theory consists of many queuing methods and associated formulas. Anyone who has taken a queuing theory class can testify to the degree of complexity of the many queuing models that have been derived for various situations.

The most commonly used model and table is Erlang B. Erlang B is based on infinite sources, LCC, and Poisson distribution that is appropriate for either exponential or constant holding times. In general, Erlang B understates the number of trunks because of the LCC assumption, but it is generally the most commonly used algorithm.

Count to infinity

A distance vector routing protocol uses infinity to know when a route is really down. Counting to infinity is good (and necessary for these protocols to work). If the hop count rises to a certain number, it is considered unreachable and removed from the routing table and not propagated further. In RIP v1, the infinity hop count was only 16. This led to quick convergence but a small network diameter.

Count to infinity helps keep false routes from propagating for non adjacent routers while split-horizon does the same for adjacent routers.


When TCP session sequence numbers are received out of sequence the detecting station will send a reset. This forces the session to abort.


If you look at the TCP header format. There is a six bit field called FLAGS. FLAGS are used for data flow and connection control. Valid flags are URG (urgent) ACK (acknowledgement) PSH (push) RST (reset) SYN (syncronize) and FIN (final) they are viewable with a decent sniffer trace of a TCP session.

Difference between Ethernet II and 802.3

802.3 represents a LENGTH field, while the Ethernet II represents the TYPE field.

Configuring a Default Route for IGRP

IGRP doesn’t recognize the command “ip route” just as an example. With IGRP, it requires the following syntax command, “ip default-network X.X.X.X“.


TACACS+ uses TCP. The entire body of TACACS+ packets is encrypted if a shred key is set on the NAS and the server. TACACS+ can log every command that is entered at the router exec command. TACACS+ can pass caller ID or called party values from NAS to the server.


ID / Protocol
06 – TCP/IP
E0 – Novell IPX
F0 – NetBIOS
42 – Spanning
04, 05, 08, 0C – SNA

Routing – administrative distance

0 – directly connected (or a static directed to a connected interface)
1 – static route (to an IP address)
5 – EIGRP summary
20 – eBGP
90 – EIGRP internal
100 – IGRP
110 – OSPF
115 – IS-IS
120 – RIP
140 – EGP
160 – ODR
170 – EIGRP external
200 – iBGP
255 – Unknown (unreachable)

Multicast addresses – All hosts on this subnet – All routers on this subnet – DVMRP routers – All OSPF routers – All OSPF DR routers – All RIPv2 routers – All (E)IGRP routers

RIP – interface summary

(config-if)# ip summary-address rip
(config-if)# no ip split-horizon
(in case it was turned on)

  1. Global – auto-summary (default = on)
  2. Interface – summary-address (over rules global config)
  3. interface – split-horizon needs to be turned off for interface summary-address to work (default = on)

RIP seed metric: Do not mistake this for administrative distance, it’s the metric (hop count) given to routes redistributed into RIP. A redistributed static has a seed metric of 1. A connected route has a seed metric of 0.


Q: In a FR environment, the central site can see all the remote sites, but the remote sites cannot see each other, what has caused the problem?
A: You need to turn off the poison reverse on the split horizon.

Q: To monitor keepalives on a serial interface
A: Use the command “debug serial interface” to monitor keepalives on the interface.

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